In recent years, as a variety of electronic equipments become compact, electronic devices to be installed inside the electronic equipments have become more compact and higher in performance. As one of the electronic devices, there is a ceramic electronic device, such as a CR built-in substrate and a multilayer ceramic capacitor, and the ceramic electronic devices have been required to be more compact and higher in performance.
To pursue a more compact ceramic electronic device having a higher capacity, there is a strong demand for making a dielectric layer thinner. Recently, a thickness of a dielectric green sheet composing a dielectric layer has become a several μm or less.
To produce a ceramic green sheet, normally, a ceramic paste composed of ceramic powder, a binder (an acrylic resin and butyral based resin, etc.), a plasticizer (phthalate esters, glycols, adipic acids, and phosphoric esters) and an organic solvent (toluene, MEK and acetone, etc.) is prepared. Next, the ceramic paste is applied to a carrier sheet (a supporting body made by PET or PP) by the doctor blade method, etc., heated and dried to produce.
Also, a method of producing by preparing a ceramic suspension wherein the ceramic powder and binder are mixed in a solvent, then, extruding by twin-screw a film-shaped molded item obtained by molding the suspension has been considered in recent years.
A method of producing a multilayer ceramic capacitor by using the ceramic green sheet explained above will be explained in detail. An internal electrode conductive paste containing electrode material powder and a binder is printed to be a predetermined pattern on the ceramic green sheet and dried to form an internal electrode pattern. After that, the green sheet is peeled from the carrier sheet and stacked by a predetermined number of layers. Here, two methods are proposed, that are a method of peeling the green sheet from the carrier sheet before stacking in layers and a method of peeling the carrier sheet after stacking in layers and adhering by compression, but the difference is not large. Finally, the stacked body is cut to be chips, so that green chips are prepared. After firing the green chips, external electrodes are formed, so that electronic devices, such as multilayer ceramic capacitors, are produced.
When producing a multilayer ceramic capacitor, an interlayer thickness of sheets formed with internal electrodes is in a range of 3 μm to 100 μm or so based on a desired capacitance required as a capacitor. Also, in a multilayer ceramic capacitor, a part not formed with internal electrodes is formed on an outer part in the stacking direction of the capacitor chip.
In such a multilayer ceramic capacitor, it was general that a binder used in the green sheet paste was a polyvinyl butyral resin having a polymerization degree of 1000 or less (Mw=50,000) (refer to the Japanese Patent Publication No. 10-67567). As the reasons, to sufficiently secure adhesiveness of ceramic green sheets at the time of stacking, to reduce surface roughness of the green sheets, to secure plasticity of the green sheets, and to reduce viscosity of slurry may be mentioned. As a plasticizer, generally, phthalic acid, adipic acid, sebacic acid, and phosphoric esters can be used, which were selected in terms of a boiling point and hazardous property, etc. for the purpose of giving plasticity.
In recent years, as electronic equipments become more compact, electronic devices to be used therein have rapidly become more compact. In multilayer electronic devices as typified by a multilayer ceramic capacitor, rapid development has been made on increasing the number of layers to be stacked and attaining a thinner interlayer thickness. To respond to the technical trends, a thickness of a green sheet, which determines the interlayer thickness, has almost become 3 μm or less to 2 μm or less. Also, along therewith, a thickness of the internal electrode layer has become 1.5 μm or less, and the number of layers to be stacked has almost become 300 or more.
To form an internal electrode on such a thin green sheet, when using the conventional printing method, there arises a problem of so called sheet-attack that a solvent included in an internal electrode paste melts the green sheet. Thus, the dry transfer method has been developed.
In the dry transfer method, first, a release layer is formed on a PET film as a supporting sheet and an internal electrode layer is printed thereon. Furthermore, to eliminate a level difference due to a thickness of the internal electrode layer, a blank pattern layer having the same thickness as that of the internal electrode layer is formed on a blank pattern portion where an electrode is not formed.
A resin layer (adhesive layer) having adhesiveness is formed on a different PET film from the PET film formed with the internal electrode layer, and transferred to the internal electrode layer and the blank pattern layer by thermo-compression bonding. Then, the PET film on the resin layer side is removed.
A dielectric green sheet is formed on a still other PET film and transferred to the resin layer by heat transfer.
In this way, a release layer and an electrode are unified with the blank pattern layer, the resin layer and the green sheet, and the result is stacked successively, consequently, stacking of thin film sheets with no sheet-attack becomes possible.
A thickness of 0.1 μm or less of an adhesive layer used in the dry transfer method is effective to prevent delamination during the binder burnout. A butyral based resin is extremely effective to obtain sufficient strength and adhesive force even in the case of such an extremely thin layer.
An internal electrode paste used in the conventional printing method is often composed of an ethyl cellulose based resin, metal powder and a solvent. However, since ethyl cellulose based resins were poor in strength and adhesiveness, there was a problem that electrode layer breaking and an adhesive defect were easily caused in the dry transfer method.
On the other hand, to make a chip capacitor to be more compact and have a larger capacity, an electrode layer has to be made thin and smooth as well as thinning a dielectric layer.
Since metal weight per one layer becomes light as the electrode becomes thinner, a metal adhering amount has to be less when forming an internal electrode layer by the printing method. It is advantageous in terms of procedure costs to decrease the metal adhering amount by lowering a metal content in the print paste in a conventional facility. However, when the solvent ratio is made high to lower the metal ratio, the paste viscosity abruptly declines and it becomes impossible for a conventional printing facility to deal with.
Also, to improve smoothness of the internal electrode layer after firing, it is efficient to make the metal filling density high in the internal electrode layer at the stage of green. To make the metal filling rate high, an amount of other component, for example a binder resin, may be decreased. However, when the binder resin is decreased, the paste viscosity declines, so that it is necessary to use a binder having high viscosity.